The shape of 2 F1− F2 suppression tuning curves reflects basilar membrane specializations in the mustached bat, Pteronotus parnellii

Abstract

Iso-suppression tuning curves (STCs) of the 2 F 1− F 2 distortion product (dp) were measured over a primary frequency range of 20 to 93 kHz in mustached bats, Pteronotus parnellii parnellii. Primary levels were chosen to produce dp levels between 0 and 7 dB SPL. At frequencies outside the ranges of 60 – 72 kHz and 90 – 93 kHz the shapes of the STCs were symmetrical or asymmetrical with a steep high frequency slope. In the vicinity of 61 kHz where a strong stimulus frequency otoacoustic emission (SFOAE) is present, the asymmetry of the STCs was inverted with a very steep low frequency slope (max. −89 dB/kHz) and a shallow high frequency slope. The inverted STCs resemble neuronal tuning curves of the same species with best frequencies at about 61 kHz. Close to 61 kHz the STCs were sharply tuned with Q 10dB values up to 177. The STC-thresholds were about 20 dB above the neuronal thresholds. Thickenings of the basilar membrane located just basal to the cochlear place of the SFOAE frequency are probably involved in creating the asymmetric STCs. Cochlear resonance at the SFOAE frequency and an increased longitudinal coupling within the thickened basilar membrane region are thought to contribute to the specialized STC shape. In the range of 90 – 93 kHz, the STCs are also sharply tuned with inverted asymmetry which is probably not due to an harmonic effect of the specialized cochlear mechanics in the 60 kHz region but may be caused by an independent mechanism.